1. Field of the Invention
This invention generally relates to a motor, a method of manufacturing a motor, and a method of magnetizing a raw member for a magnet in a motor to make the magnet. This invention particularly relates to a magnetizing method, and a motor and its manufacturing method which are designed so that a rotor and a stator magnetically attract each other to provide improved rotation characteristics of the motor.
2. Description of the Related Art
A typical optical disc drive contains a motor for rotating and driving an optical disc. A general laser beam printer (LBP) contains a motor for rotating and driving a polygon mirror. Such motors are required to have extremely stable rotation characteristics.
There are small-sized portable optical disc devices having optical disc drives respectively. In general, such an optical disc device is required to normally operate regardless of the position or attitude of a device body. The position of a motor in the optical disc device depends on that of the device body.
In some cases, a motor in an LBP is in an unusual position to implement the miniaturization of the LBP body. A first example of the unusual position is such that the rotary shaft (the spindle) of the motor tilts relative to the main walls of the LBP casing. A second example of the unusual position is called an inverted position where the rotary shaft of the motor may fall out of its support bearing due to its weight (the rotor is closer to the ground than the stator is).
Accordingly, motors are requested which exhibit stable rotation performances regardless of their positions or attitudes.
In the event that the rotor of a motor in an optical disc drive develops a whirl (an undesirable deflection) due to unsteady support of the rotary shaft of the motor or floats in a direction of slipping out of a related bearing during operation of the motor, the ability of the optical disc drive to read and write information from and on an optical disc is impaired. When a motor in an LBP has a similar problem, the printing performance of the LBP is decreased.
Japanese patent application publication number 2002-281712 discloses a disc driving motor designed to prevent the occurrence of a whirl. The motor in Japanese application 2002-281712 includes a magnet which is designed to attract the rotor of the motor toward the stator to enhance the rotation characteristics of the motor.
Specifically, in the motor of Japanese application 2002-281712, an annular back yoke is fixed to a step on an outer circumferential portion of a bearing holder in a stator side, and an annular bias magnet is bonded to a surface of the back yoke which faces a rotor side. The circumferential-direction magnetic pole width (angular range) of the bias magnet is made uneven. Thereby, an edgewise pressure (a side pressure) is applied to a spindle. A rotor yoke in the rotor side which is opposed to the bias magnet is magnetically attracted toward the stator side so that an undesirable clearance between the spindle and a related bearing can be removed. Therefore, it is possible to prevent the occurrence of a whirl. Furthermore, it is possible to stably maintain the rotation of the rotor at high speeds.
As will be explained later, it is desirable to improve the motor of Japanese application 2002-281712.